Disc drive apparatus, and method for recognizing cd and dvd

ABSTRACT

A method is described for measuring the thickness of an optical disc and determining whether the disc is a CD or a DVD. The optical lens is caused to move towards the disc with a substantially constant speed, with the optical beam switched on, and the focal error signal (FES) is analyzed. By timing the S-shaped curve ( 62 ), the actual value of said speed is determined. By timing different reflections ( 72, 73 ) from the disc, taking into account the measured speed, the thickness of the disc is determined. An important advantage is that the outcome of the thickness measurement is substantially independent from the actual value of said speed, and thus independent from the actual sensitivity of the actuator.

The present invention relates in general to a disc drive apparatus forwriting/reading information into/from an optical storage disc;hereinafter, such disc drive apparatus will also be indicated as“optical disc drive”. The present invention relates further to a methodfor identifying the type of a disc inserted in the disc drive apparatus.

As is commonly known, optical discs have been developed according todifferent formats. By way of example, CD and DVD are mentioned here, butthe gist of the present invention is also applicable to other disctypes. Conventionally, disc drives have been developed as dedicateddevices, i.e. suitable for one disc type only. Thus, for instance,optical disc drives of CD type have been developed on the one hand, andoptical disc drives of DVD type have been developed on the other hand.Such dedicated disc drives are suitable for one type of optical disconly; if the wrong type of disc is inserted in such drive, the discdrive can not handle the disc and responds with an error message. Inother words, such dedicated disc drives know what type of disc toexpect, “wrong types” are handled according to the format of theexpected disc type.

More recently, disc drives have been developed which are capable ofhandling two (or more) different types of disc. Such type of disc drivewill be indicated as multiple-type drive. As a specific example, amultiple-type drive for handling CDs and DVDs will be described in thefollowing, but it is to be noted that such description is not intendedto restrict the protective scope of the present invention to thisexample since the gist of the present invention is also applicable toother types of disc.

Since a multiple-type drive may expect a disc to be any of two (or more)different types of disc, it needs to ascertain the type of disc when anew disc is inserted, in order to be able to handle the disc with thecorrect format.

Thus, in a multiple-type drive, there is a need for a method andapparatus for determining disc type.

An important characteristic distinguishing CDs and DVDs from each otheris the thickness of the disc. A CD has a thickness of 1.2 mm whereas aDVD has a thickness of 0.6 mm. Thus, a method to recognize a CD and aDVD, or at least to distinguish between CD and DVD, has been developedon the basis of measuring the thickness of the disc: if the thicknessappears to be approximately 0.6 mm (or less than a reference value, e.g.0.9 mm) it is concluded that the disc is a DVD, whereas, if thethickness appears to be approximately 1.2 mm (or more than a referencevalue, e.g. 0.9 mm) it is concluded that the disc is a CD.

U.S. Pat. No. 6,061,318 discloses a method for discriminating disc typeon the basis of the thickness of the disc. The focal actuator iscontrolled with a ramping voltage to axially displace the objective lenssuch that the focal point of the laser beam is axially displaced towardsthe disc, and the focal error signal is monitored. A characteristic ofthe focal error signal indicates when the focal point reaches thesurface of the disc at a first moment in time, and when the focal pointreaches an information layer at a second moment in time. The thicknessof the disc may be calculated from the time distance between the firstand the second moment in time, taking the displacement speed of theoptical lens into account, which depends on the slope of the actuatordriving voltage.

A problem with this prior art method is that the displacement speed ofthe optical lens is not accurately known, because the actuatorsensitivity, i.e. displacement as a function of control voltage (mm/V),is not an accurately known constant. Typically, this sensitivity rangesfrom 0.65 mm/V to 1.3 mm/V. Even for one specific actuator, thissensitivity may vary with age of the actuator, and condition (e.g.temperature) of the actuator.

A main objective of the present invention is to overcome this problem.

U.S. Pat. No. 6,285,641 also discloses a method for discriminating disctype on the basis of the thickness of the disc, in a disc driveapparatus wherein the laser beam has two focal points axially displacedwith respect to each other over a known distance. The focal actuator iscontrolled with a ramping voltage to axially displace the objectivelens; the actual speed of the objective lens is determined from the timedifference between two signal peaks originating from said two focalpoints. However, this method can only be executed if the laser beam hastwo focal points.

Specifically, the present invention aims to provide a more reliable discrecognition method, which is capable of being executed in a disc driveapparatus wherein the laser beam has only one focal point.

More specifically, the present invention aims to provide a discrecognition method where the dependency of the outcome on actuatorsensitivity is eliminated or at least reduced.

According to an important aspect of the present invention, a method forcalibrating the focal actuator is provided, wherein the time differencebetween the occurrence of a first characteristic feature of the focalerror signal and the occurrence of a second characteristic feature ofthe focal error signal is taken into account. Preferably, said firstcharacteristic feature and second characteristic feature are the maximumvalue and the minimum value, respectively, of the focal error signal,based on the insight that these two extremities have a fixed distancedetermined by the design of the optical system.

In a first special aspect, a method for measuring the speed of the focalactuator is provided.

In a second special aspect, a method for measuring the sensitivity ofthe focal actuator is provided.

In a third special aspect, a method for measuring the thickness of anoptical disc is provided.

These and other aspects, features and advantages of the presentinvention will be further explained by the following description withreference to the drawings, in which same reference numerals indicatesame or similar parts, and in which:

FIG. 1 schematically illustrates some relevant components of an opticaldisc drive apparatus;

FIG. 2 schematically illustrates an optical detector;

FIG. 3 schematically illustrates astigmatism;

FIG. 4A schematically illustrates an S-shaped curve;

FIG. 4B-D schematically illustrate the shape of a light spot on anoptical detector;

FIG. 5 schematically illustrates optical signals as a function of time.

FIG. 1 schematically illustrates an optical disc drive apparatus 1,suitable for storing information on or reading information from anoptical storage disc 2, typically a DVD or a CD. The optical disc 2comprises at least one track, either in the form of a continuous spiralor in the form of multiple concentric circles, of storage space whereinformation may be stored in the form of a data pattern. The opticaldisc may be read-only type, where information is recorded duringmanufacturing, which information can only be read by a user. The opticaldisc may also be a writable type, where information may be stored by auser. Since the technology of optical discs in general, the way in whichinformation can be stored in an optical disc, and the way in whichoptical data can be read from an optical disc, is commonly known, it isnot necessary here to describe this technology in more detail.

For rotating the disc 2, the disc drive apparatus 1 comprises a motor 4fixed to a frame (not shown for sake of simplicity), defining a rotationaxis 5. For receiving and holding the disc 2, the disc drive apparatus 1may comprise a turntable or clamping hub 6, which in the case of aspindle motor 4 is mounted on the spindle axle 7 of the motor 4.

The disc drive apparatus 1 further comprises an optical system 30 forscanning tracks of the disc 2 with an optical beam. More specifically,in the exemplary arrangement illustrated in FIG. 1, the disc driveapparatus 1 is a multiple-type drive designed for handling two types ofdisc, i.e. CD as well as DVD for example. The optical system 30comprises a first light beam generating means 31 and a second light beamgenerating means 41, each typically a laser such as a laser diode, eacharranged to generate a first light beam 32 and a second light beam 42,respectively. In the following, different sections of the optical pathof a light beam 32, 42 will be indicated by a character a, b, c, etcadded to the reference numeral 32, 42, respectively. It is noted that,in a disc drive apparatus designed for handling only one type of disc,i.e. only CD for example, typically only one laser diode will bepresent.

The first light beam 32 passes a first beam splitter 43, a second beamsplitter 33, a collimator lens 37 and an objective lens 34 to reach(beam 32 b) the disc 2. The first light beam 32 b reflects from the disc2 (reflected first light beam 32 c) and passes the objective lens 34,the collimator lens 37 and the second beam splitter 33 (beam 32 d) toreach an optical detector 35.

The second light beam 42 is reflected by a mirror 44, passes the firstbeam splitter 43, and then follows an optical path comparable with theoptical path of the first light beam 32, indicated by reference numerals42 b, 42 c, 42 d.

The objective lens 34 is designed to focus one of the two light beams 32b, 42 b in a focal spot F on an information layer (not shown for sake ofsimplicity) of the disc 2, which spot F normally is circular. Forexplaining the present invention, it will be assumed in the followingthat only the first laser 31 is operated and that the second laser 41 isOFF.

During operation, the light beam should remain focussed on the recordinglayer. To this end, the objective lens 34 is arranged axiallydisplaceable, and the optical disc drive apparatus 1 comprises a focalactuator 52 arranged for axially displacing the objective lens 34 withrespect to the disc 2. Since axial actuators are known per se, whilefurther the design and operation of such axial actuator is no subject ofthe present invention, it is not necessary here to discuss the designand operation of such focal actuator in great detail.

It is noted that means for supporting the objective lens with respect toan apparatus frame, and means for axially displacing the objective lens,are generally known per se. Since the design and operation of suchsupporting and displacing means are no subject of the present invention,it is not necessary here to discuss their design and operation in greatdetail.

The disc drive apparatus 1 further comprises a control circuit 90 havingan output 94 coupled to a control input of the focal actuator 52, and aread signal input 91 for receiving a read signal S_(R) from the opticaldetector 35. The control circuit 90 is designed to generate at itsoutput 94 a control signal S_(CF) for controlling the focal actuator 52.

FIG. 2 illustrates that the optical detector 35 comprises a plurality ofdetector segments, in this case four detector segments 35 a, 35 b, 35 c,35 d, capable of providing individual detector signals A, B, C, D,respectively, indicating the amount of light incident on each of thefour detector quadrants, respectively. A centre line 36, separating thefirst and fourth segments 35 a and 35 d from the second and thirdsegments 35 b and 35 c, has a direction corresponding to the trackdirection. Since such four-quadrant detector is commonly known per se,it is not necessary here to give a more detailed description of itsdesign and functioning.

FIG. 2 also illustrates that the read signal input 91 of the controlcircuit 90 actually comprises four inputs 91 a, 91 b, 91 c, 91 d forreceiving said individual detector signals A, B, C, D, respectively. Thecontrol circuit 90 is designed to process said individual detectorsignals A, B, C, D, in order to derive data and control informationtherefrom, as will be clear to a person skilled in the art. Forinstance, a data signal S_(D) can be obtained by summation of allindividual detector signals A, B, C, D according toS _(D) =A+B+C+D  (1)Further, a focal error signal S_(FE) can be obtained by summation of thesignals A and C from one pair of individual detector segments 35 a and35 c diagonally opposite to each other, summation of the signals B and Dfrom the other pair of individual detector segments 35 b and 35 ddiagonally opposite to each other, and taking the difference of thesetwo summations, according toS _(FE)=(A+C)−(B+D)  (2a)In order to compensate light intensity variations of the beam as awhole, this error signal can be normalized by division by the datasignal to obtain a normalized focal error signal FES according toFES=S _(FE) /S _(D)  (2b)The light beam 32 d is subject to astigmatism. This may for instance becaused by the second beam splitter 33, which, in the example depicted inFIG. 1, is implemented as a tilted beam splitter plate. Alternatively,or additionally, it is also possible that the optical path includes anadditional optical element, not shown in FIG. 1 for sake of clarity,located in front of the detector 35, for deliberately introducingastigmatism, as will be clear to a person skilled in the art.Astigmatism means that, instead of having one focal point where alllight rays meet, a converging light beam has two elongate focal spots,axially displaced with respect to each other, and oriented perpendicularto each other, as illustrated in FIG. 3. FIG. 3 shows an optical systemgenerally indicated at 100 having an optical centre 101 and an opticalaxis 102. An object point is indicated at 103, located at a distancefrom the optical axis. A principal ray 104 emerging from the objectpoint 103 passes the optical centre 101 without refraction.

A first main plane 110, indicated as tangential plane, is defined by theoptical axis 102 and the object point 103. Rays emerging from the objectpoint 103 and located in this plane are indicated as tangential rays111. These tangential rays 111 are refracted by the optical system 100such as to be focused in a tangential focal spot 112.

A second main plane 120, indicated as sagittal plane, is defined by theprincipal ray 104 and extends perpendicular to the tangential plane.Rays emerging from the object point 103 and located in this plane areindicated as sagittal rays 121. These sagittal rays 121 are refracted bythe optical system 100 such as to be focused in a sagittal focal spot122.

Astigmatism means that the sagittal focal spot 122 does not coincidewith the tangential focal spot 112. In the example of FIG. 3, the axialdistance from the sagittal focal spot 122 to the optical centre 101 islarger than the axial distance from the tangential focal spot 112 to theoptical centre 101. It can be shown that the axial distance between thesagittal focal spot 122 and the tangential focal spot 112 dependssubstantially only on optical parameters of the optical system, such as,in the case of an optical system 30 of a disc drive, the focal length ofthe objective lens, the focal length of the collimator lens, therefractive index of the beam splitter 33, the thickness of the beamsplitter 33, the incident angle between reflected beam 32 c and the beamsplitter 33.

The sagittal focal spot 122, hereinafter also indicated as F_(S), is nota point in space. It can clearly be seen from FIG. 3 that, in thesagittal focal spot F_(S), all sagittal rays 121 are focussed but alltangential rays 111 are beyond their focal point and are divergingagain, such that the sagittal focal spot F_(S) has an elongate shape. Inan ideal case, the sagittal focal spot F_(S) has the shape of a linesegment located in the tangential plane 110 and perpendicular to theoptical axis 102.

Likewise, the tangential focal spot 112, hereinafter also indicated asF_(T), is not a point in space. It can clearly be seen from FIG. 3 that,in the tangential focal spot F_(T), all tangential rays 111 are focussedbut all sagittal rays 121 have not yet reached their focal point and arestill converging, such that the tangential focal spot F_(T) has anelongate shape. In an ideal case, the tangential focal spot F_(T) hasthe shape of a line segment located in the sagittal plane 120 andperpendicular to the optical axis 102.

Thus, the elongate tangential focal spot F_(T) and the elongate sagittalfocal spot F_(S) are perpendicular to each other, having a fixed axialdistance which will be indicated hereinafter as astigmatic focaldistance ΔF. Approximately halfway between the tangential focal spotF_(T) and the sagittal focal spot F_(S), the light beam has asubstantially circular cross-section in a so-called “circle of leastconfusion” 109, hereinafter also indicated as circular focal pointF_(C).

FIGS. 4A-D and 5 illustrate the optical signals obtained when the focusactuator 52 displaces the objective lens 34. In FIG. 4A, the line 61indicates a control voltage S_(CF) applied by the control circuit 90 tothe focus actuator 52, and the curve 62 indicates the normalized focalerror signal FES, as a function of time. As the control voltageincreases, the objective lens 34 is moved towards the disc 2. Initially,the focus points F_(S) and F_(T) are well below the information layer ofthe disc, and the detector 35 receives only little reflected light,while further the cross-sectional shape of the optical spot on thedetector 35 is more or less circular. When the sagittal focus pointF_(S) approaches the information layer, FES increases, and reaches amaximum at time t_(S) when the sagittal focus point F_(S) coincides withthe information layer; FIG. 4B illustrates the shape of the optical spoton the detector 35 for this situation.

With a further increase of the control voltage S_(CF), the shape of theoptical spot on the detector 35 becomes more and more circular, until attime t_(CR) the circular focal point F_(C) coincides with theinformation layer, FIG. 4C illustrates the circular shape of the opticalspot on the detector 35 for this situation. At this moment, FES equalszero. This condition is considered to be the optimum focus condition forreading or writing optical information from/to disc, and, normally, afocus servo system is adapted to control the focus actuator to maintainthe objective lens in this condition.

With a still further increase of the control voltage S_(CF), theabsolute value of FES increases again, but now FES has opposite signbecause the optical spot on the detector 35 becomes elongate in anotherdirection. At time t_(T), FES reaches a maximum negative value, orminimum, when the tangential focus point F_(T) coincides with theinformation layer; FIG. 4D illustrates the shape of the optical spot onthe detector 35 for this situation.

With a further increase of the control voltage S_(CF), the absolutevalue of FES decreases again.

In view of its shape, curve 62 is also indicated as “S-shaped curve”.

FIG. 5 is a graph similar to FIG. 4A but on a larger time scale. ThisFIG. 5 shows that, at lower values of the control voltage S_(CF), asecond S-shaped curve 63 is observed, now caused by reflection of thelight beam 32 b by the lower surface of the disc. The time when thecircular focal point F_(C) coincides with this lower disc surface, i.e.when the focal error signal FES crosses zero, is indicated as t_(CS).Usually, the second S-shaped curve 63 corresponding with the lower discsurface has a smaller amplitude than the first S-shaped curve 62corresponding with the information layer, as shown in FIG. 5.

FIG. 5 also shows the low frequency part of the data signal S_(D) (alsoknown as central aperture signal CA) for the beam reflected from thelower disc surface and the beam reflected from the information layer,respectively, as curves 73 and 72, respectively. Usually, the secondS_(D)-curve 73 corresponding with the lower disc surface has a smalleramplitude than the first S_(D)-curve 72 corresponding with theinformation layer, as shown in FIG. 5. As can be seen from FIG. 5, theS_(D)-curves 73 and 72 have maximum values at times t_(CS) and t_(CR),respectively.

Since the astigmatic focal distance ΔF is an apparatus constant, thespeed V of the optical lens 34 can be calculated from the time intervalΔt=t_(T)−t_(S), according to formula (3):V=ΔF/Δt  (3)assuming, of course, that the speed V of the optical lens 34 is constantduring said time interval.

With reference to FIG. 5, the thickness D of the disc 2, or moreprecisely the distance between lower disc surface and information layer,can be calculated from the speed V of the optical lens 34 and the timeinterval Δt_(C)=t_(CR)−t_(CS), according to formula (4):D=V*Δt _(C)  (4)under the assumption that the speed V of the optical lens 34 remainsconstant during this time interval, or at least that the average speedmeasured during the time interval Δt=t_(T)−t_(S) is a sufficientlyaccurate approximation of the average speed occurring during the timeinterval from t_(CS) to t_(CR).

In a first embodiment, the control circuit 90 may calculate the durationof the time interval Δt_(C), i.e. determine the times t_(CS) and t_(CR),on the basis of the focal error signal FES, i.e. by determining when thezero-crossings of the focal error signal FES occur.

In a second embodiment, the control circuit 90 may calculate theduration of the time interval Δt_(C), i.e. determine the times t_(CS)and t_(CR), on the basis of the low frequency part of the data signalS_(D), i.e. by determining when the peaks of the low frequency part ofthe data signal S_(D) occur. In this respect it is noted thatdetermining zero-crossings may be more accurate than determining thetiming of a peak, but on the other hand the second S-shaped curve 63corresponding to the lower disc surface is very small, so that using thelow frequency part of the data signal S_(D) is more convenient.

The control circuit 90 is now capable of deciding whether the disc 2 isa CD or a DVD and handling the disc in accordance with the correctformat. For instance, by comparing the calculated thickness D with asuitable reference value D_(REF), for instance D_(REF)=0.9 mm, thecontrol circuit 90 may decide that the disc 2 is a CD if D>D_(REF) andthat the disc 2 is a DVD if D<D_(REF).

Thus, the present invention provides an improved method for measuringthe thickness of an optical disc and determining whether the disc is aCD or a DVD. The optical lens 34 is caused to move towards the disc 2with a substantially constant speed, with the optical beam 32 switchedon, and the focal error signal is analyzed. By timing the S-shapedcurve, the actual value of this speed V is determined. By timingdifferent reflections from the disc, taking into account the measuredspeed V, the thickness of the disc is determined. An important advantageis that the outcome of the thickness measurement is substantiallyindependent from the actual value of said speed V, and thus independentfrom the actual sensitivity of the actuator.

Although the present invention is intended to provide a method for arelative accurate determination of the thickness of an optical disc, andto provide a method for a reliable determination whether the disc is aCD or a DVD, an embodiment of the present invention also provides amethod for measuring the speed V of the optical lens 34 (see formula 3).

Further, an embodiment of the present invention also provides a methodfor measuring the sensitivity γ of the actuator 52, which is definedaccording to formula 5:V=γ*d(S _(CF))/dt  (5)S_(CF) being the (voltage of the) control signal from the controlcircuit 90 to the actuator 52. After having calculated speed V inaccordance with formula 1, the control circuit 90, knowing thetime-derivative of its control signal, can calculate the sensitivity γby dividing speed V by said time-derivative according to formula 6:γ=V/(d(S _(CF))/dt)  (6)On the other hand, if it is only desired to determine the type of disc,it is not necessary to actually calculate the speed V of the opticallens 34. By combining formulas 3 and 4, it is sufficient to calculatethe actual value of a disc type parameter α, defined in accordance withformula 7:α=Δt _(C) /Δt  (7)It is possible to determine in advance the expected value of such disctype parameter α for a CD and for a DVD. For example, if for a certainoptical system the astigmatic focal distance ΔF is equal to 10 μm, thenα_(CD)=120 and α_(DVD)=60. Thus, it is possible to define in advance areference value α_(REF) for the disc type parameter α, for instanceα_(REF)=90. Thus, the control circuit 90 may compare the actual value ofthe disc type parameter α with this reference value α_(REF), and maydecide that the disc 2 is a CD if α>α_(REF) and that the disc 2 is a DVDif α<α_(REF).

It should be clear to a person skilled in the art that the presentinvention is not limited to the exemplary embodiments discussed above,but that various variations and modifications are possible within theprotective scope of the invention as defined in the appending claims.

For instance, it is to be noted that the present invention is notlimited to a multiple-type drive. The present invention can also beapplied in a dedicated disc drive, intended for one type of disc only,to determine whether a wrong type of disc has been inserted.

Further, the present invention can likewise be used to differentiate forinstance Blu-Ray discs from for instance DVD discs and CD discs.

Further, the present invention can likewise be exercised by moving theoptical lens away from the disc, starting at a starting point close tothe disc. Normally, however, when a new disc is inserted, or when thedisc drive apparatus is switched on or initialized, the optical lenswill initially be in a parking position at a relatively large distancefrom the disc. Nonetheless, after having measured the thickness of thedisc with the method as explained above, a similar measurement can berepeated with the lens moving in the opposite direction.

In the above, a measurement for determining the speed V of the opticallens has been explained by measuring the time interval between theoccurrence of two characteristic events, i.e. the maximum value and theminimum value, associated with the sagittal focal point and thetangential focal point, of one S-shaped curve only. However, asexplained with reference to FIG. 5, two S-shaped curves are observedwhen measuring the thickness of the disc. In fact, in association witheach reflective layer of the disc, respective S-shaped curves are to beexpected. The speed measurement can be performed in conjunction witheach of said S-shaped curves. Thus, in the example described, the speedV can be calculated for the S-curve 63 associated with the lower surfaceof the disc, and the speed V can also be calculated for the S-curve 62associated with the information layer of the disc. Thus, two measuringresults are obtained. It is possible that these two measuring resultsare compared, and that the measurements are considered as being correctonly if they correspond within a certain predetermined tolerance.However, it is also possible that the average speed of the objectivelens is calculated as being the mathematical average of the twomeasuring results, and that this average speed is used in formula 4.

In the above, the present invention has been explained by taking intoaccount two reflections from two reflective layers, i.e. the discsurface on the one hand and an information layer on the other hand.However, it is also possible that a disc has multiple informationlayers. In such case, the method proposed by the present invention canlikewise be exercised by taking into account the respective reflectionsfrom such information layers for determining the number and/or locationsand/or mutual distances of such multiple information layers, andeventually determining the type of disc from the outcome of suchdetermination.

In the above, the present invention has been explained by taking intoaccount the time interval Δt_(C)=t_(CR)−t_(CS) between zero-crossings ofthe focal error signal. However, instead of using the zero-crossings ofthe focal error signal, it is also possible to use other characteristicevents of the focal error signal, such as the extreme values (i.e.t_(S), t_(T)).

In the above, the present invention has been explained by discussing aprocessing of the normalized focal error signal FES. Although it ispreferred to use the normalized focal error signal FES, indeed, such is,however, not essential, and the present invention can also be exercisedby using the non-normalized focal error signal S_(FE) according toformula 2b, because the timing of the characteristic events of thissignal (i.e. maximum, minimum, zero-crossing) is not influenced bynormalization.

In the above, with reference to FIG. 3, astigmatism has been explainedby assuming that the focal distance of the sagittal focus point F_(S) islarger than the focal distance of the tangential focus point F_(T).However, the same explanation applies, mutatis mutandis, if the focaldistance of the sagittal focus point F_(S) is smaller than the focaldistance of the tangential focus point F_(T).

1. Method for determining axial speed (V) of an optical lens (34) of anoptical disc drive (1), wherein: a light beam (32) is generated,directed towards the optical disc (2), and caused to reflect from theoptical disc (2), wherein the light beam passes said optical lens (34);the reflected light beam (32 d) is received by an optical detector (35);an output signal (S_(R)) from said optical detector (35) is processed toderive therefrom a focal error signal (FES); said optical lens (34) iscaused to move axially with respect to said optical disc (2); anS-shaped curve (62) of the focal error signal (FES) is timed; and theaxial speed (V) of the optical lens (34) is calculated on the basis ofthe timing result of the S-shaped curve (62).
 2. Method according toclaim 1, wherein the axial speed (V) of the optical lens (34) iscalculated according to the formula:V=ΔF/|t _(T) −t _(S)| t_(S) being the time of occurrence of a firstcharacteristic event of the focal error signal (FES); t_(T) being thetime of occurrence of a second characteristic event of the focal errorsignal (FES); and ΔF being the spatial axial distance between twophysical characteristics of said light beam (32) associated with saidfirst and second characteristic events, respectively.
 3. Methodaccording to claim 2, wherein: said first characteristic event is amaximum value of the focal error signal (FES); said secondcharacteristic event is a minimum value of the focal error signal (FES);ΔF being the astigmatic focal distance of said light beam (32). 4.Method for determining the sensitivity (γ) of a focal actuator (52) ofan optical disc drive (1), wherein: a sloping control signal (S_(CF))having a substantially constant slope is applied to the focal actuator(52) such as to cause an optical lens (34) to move axially with respectto an optical disc (2); a light beam (32) is generated, directed towardsthe optical disc (2), and caused to reflect from an optical disc (2),wherein the light beam passes said optical lens (34); the reflectedlight beam (32 d) is received by an optical detector (35); an outputsignal (S_(R)) from said optical detector (35) is processed to derivetherefrom a focal error signal (FES); an S-shaped curve (62) of thefocal error signal (FES) is timed; and the sensitivity (γ) of the focalactuator (52) is calculated on the basis of the timing result of theS-shaped curve (62).
 5. Method according to claim 4, wherein thesensitivity (γ) of the focal actuator (52) is calculated according tothe formula:γ=ΔF/(|t _(T) −t _(S) |*d(S _(CF))/dt) d(S_(CF))/dt being thetime-derivative of the control signal S_(CF); t_(S) being the time ofoccurrence of a first characteristic event of the focal error signal(FES); t_(T) being the time of occurrence of a second characteristicevent of the focal error signal (FES); and ΔF being the spatial axialdistance between two physical characteristics of said light beam (32)associated with said first and second characteristic events,respectively.
 6. Method according to claim 5, wherein: said firstcharacteristic event is a maximum value of the focal error signal (FES);said second characteristic event is a minimum value of the focal errorsignal (FES); ΔF being the astigmatic focal distance of said light beam(32).
 7. Method for determining the distance (D) between two reflectivelayers of an optical disc (2), wherein: a light beam (32) is generated,directed towards the optical disc (2), and caused to reflect from theoptical disc (2), wherein the light beam passes an optical lens (34);the reflected light beam (32 d) is received by an optical detector (35);an output signal (S_(R)) from said optical detector (35) is processed toderive therefrom a focal error signal (FES); said optical lens (34) iscaused to move axially with respect to said optical disc (2); theoccurrence of characteristic events of S-shaped curves (62; 63) of thefocal error signal (FES), associated with said two layers, is timed; atleast one S-shaped curve (62) of the focal error signal (FES) is timed;and the distance (D) between said two reflective layers is calculated onthe basis of the timing result of the S-shaped curve (62) on the onehand and on the other hand on the basis of the timing result of thecharacteristic events of said S-shaped curves (62; 63).
 8. Methodaccording to claim 7, wherein an axial speed (V) of the optical lens(34) is calculated according to the formula:V=ΔF/|t _(T) −t _(S)| t_(S) being the time of occurrence of a firstcharacteristic event of said at least one S-shaped curve (62) of thefocal error signal (FES); t_(T) being the time of occurrence of a secondcharacteristic event of the same S-shaped curve (62) of the focal errorsignal (FES); and ΔF being the spatial axial distance between twophysical characteristics of said light beam (32) associated with saidfirst and second characteristic events, respectively.
 9. Methodaccording to claim 8, wherein: said first characteristic event is amaximum value of the S-shaped curve (62) of the focal error signal(FES); said second characteristic event is a minimum value of the sameS-shaped curve (62) of the focal error signal (FES); ΔF being theastigmatic focal distance of said light beam (32).
 10. Method accordingto claim 8, wherein the distance (D) is calculated in accordance withthe formula:D=V*Δt _(C) Δt_(C)=t_(CR)−t_(CS) being the time interval between saidcharacteristic events of said two S-shaped curves (62; 63).
 11. Methodaccording to claim 10, each of said characteristic events of said twoS-shaped curves (62; 63) being the zero-crossing of the correspondingS-shaped curve (62; 63).
 12. Method according to claim 7, wherein thedistance (D) is calculated according to the formula:D=ΔF*Δt _(C) /|t _(T) −t _(S)| t_(S) being the time of occurrence of afirst characteristic event of said at least one S-shaped curve (62) ofthe focal error signal (FES); t_(T) being the time of occurrence of asecond characteristic event of the same S-shaped curve (62) of the focalerror signal (FES); ΔF being the spatial axial distance between twophysical characteristics of said light beam (32) associated with saidfirst and second characteristic events, respectively; andΔt_(C)=t_(CR)−t_(CS) being the time interval between said characteristicevents of said two S-shaped curves (62; 63).
 13. Method according toclaim 12, wherein: said first characteristic event of said S-shapedcurve (62) is a maximum value of this S-shaped curve (62); said secondcharacteristic event of the same S-shaped curve (62) is a minimum valueof the same S-shaped curve (62); ΔF being the astigmatic focal distanceof said light beam (32); and each of said characteristic events of saidtwo S-shaped curves (62; 63) being the zero-crossing of thecorresponding S-shaped curve (62; 63).
 14. Method for recognizing type(CD; DVD) of an optical disc, wherein: a light beam (32) is generated,directed towards the optical disc (2), and caused to reflect from theoptical disc (2), wherein the light beam passes an optical lens (34);the reflected light beam (32 d) is received by an optical detector (35);an output signal (S_(R)) from said optical detector (35) is processed toderive therefrom a focal error signal (FES); said optical lens (34) iscaused to move axially with respect to said optical disc (2); theoccurrence of characteristic events of S-shaped curves (62; 63) of thefocal error signal (FES), associated with said two layers, is timed; atleast one S-shaped curve (62) of the focal error signal (FES) is timed;wherein a disc type parameter (α) is calculated in accordance with theformula:α=Δt _(C)/(t _(T) −t _(S)) t_(S) being the time of occurrence of a firstcharacteristic event of said at least one S-shaped curve (62) of thefocal error signal (FES); t_(T) being the time of occurrence of a secondcharacteristic event of the same S-shaped curve (62) of the focal errorsignal (FES); Δt_(C)=t_(CR)−t_(CS) being the time interval between saidcharacteristic events of said two S-shaped curves (62; 63); wherein themeasured parameter value (α) is compared with a predetermined referencevalue (α_(REF)), and wherein it is decided that the optical disc is of afirst type (CD) if the measured parameter value is larger than saidreference value (α_(REF)), and that the optical disc is of a second type(DVD) if the measured parameter value is smaller than said referencevalue (α_(REF)).
 15. Method according to claim 14, wherein: said firstcharacteristic event is a maximum value of the S-shaped curve (62) ofthe focal error signal (FES); said second characteristic event is aminimum value of the same S-shaped curve (62) of the focal error signal(FES); and each of said characteristic events of said two S-shapedcurves (62; 63) being the zero-crossing of the corresponding S-shapedcurve (62; 63).
 16. Method for determining the distance (D) between tworeflective layers of an optical disc (2), wherein: a light beam (32) isgenerated, directed towards the optical disc (2), and caused to reflectfrom the optical disc (2), wherein the light beam passes an optical lens(34); the reflected light beam (32 d) is received by an optical detector(35); an output signal (S_(R)) from said optical detector (35) isprocessed to derive therefrom a focal error signal (FES) and a datasignal (S_(D)); said optical lens (34) is caused to move axially withrespect to said optical disc (2); the occurrence of characteristicevents of data signal curves (72; 73), associated with said two layers,is timed; at least one S-shaped curve (62) of the focal error signal(FES) is timed; and the distance (D) between said two reflective layersis calculated on the basis of the timing result of the S-shaped curve(62) on the one hand and on the other hand on the basis of the timingresult of the characteristic events of said data signal curves (72; 73).17. Method according to claim 16, wherein an axial speed (V) of theoptical lens (34) is calculated according to the formula:V=ΔF/|t _(T) −t _(S)| t_(S) being the time of occurrence of a firstcharacteristic event of said at least one S-shaped curve (62) of thefocal error signal (FES); t_(T) being the time of occurrence of a secondcharacteristic event of the same S-shaped curve (62) of the focal errorsignal (FES); and ΔF being the spatial axial distance between twophysical characteristics of said light beam (32) associated with saidfirst and second characteristic events, respectively.
 18. Methodaccording to claim 17, wherein: said first characteristic event is amaximum value of the S-shaped curve (62) of the focal error signal(FES); said second characteristic event is a minimum value of the sameS-shaped curve (62) of the focal error signal (FES); ΔF being theastigmatic focal distance of said light beam (32).
 19. Method accordingto claim 17, wherein the distance (D) is calculated in accordance withthe formula:D=V*Δt _(C) Δt_(C)=t_(CR)−t_(CS) being the time interval between saidcharacteristic events of said data signal curves (72; 73).
 20. Methodaccording to claim 19, each of said characteristic events of said datasignal curves (72; 73) being the peak of the corresponding curve (72;73) of the low frequency part of the data signal.
 21. Method accordingto claim 16, wherein the distance (D) is calculated according to theformula:D=ΔF*Δt _(C) /|t _(T) −t _(S)| t_(S) being the time of occurrence of afirst characteristic event of said at least one S-shaped curve (62) ofthe focal error signal (FES); t_(T) being the time of occurrence of asecond characteristic event of the same S-shaped curve (62) of the focalerror signal (FES); ΔF being the spatial axial distance between twophysical characteristics of said light beam (32) associated with saidfirst and second characteristic events, respectively; andΔt_(C)=t_(CR)−t_(CS) being the time interval between said characteristicevents of said data signal curves (72; 73).
 22. Method according toclaim 21, wherein: said first characteristic event of said S-shapedcurve (62) is a maximum value of this S-shaped curve (62); said secondcharacteristic event of the same S-shaped curve (62) is a minimum valueof the same S-shaped curve (62); ΔF being the astigmatic focal distanceof said light beam (32); and each of said characteristic events of saiddata signal curves (72; 73) being the peak of the corresponding curve(72; 73) of the low frequency part of the data signal.
 23. Method forrecognizing type (CD; DVD) of an optical disc, wherein: a light beam(32) is generated, directed towards the optical disc (2), and caused toreflect from the optical disc (2), wherein the light beam passes anoptical lens (34); the reflected light beam (32 d) is received by anoptical detector (35); an output signal (S_(R)) from said opticaldetector (35) is processed to derive therefrom a focal error signal(FES); said optical lens (34) is caused to move axially with respect tosaid optical disc (2); the occurrence of characteristic events of datasignal curves (72; 73), associated with said two layers, is timed; atleast one S-shaped curve (62) of the focal error signal (FES) is timed;wherein a disc type parameter (a) is calculated in accordance with theformula:α=Δt _(C)/(t _(T) −t _(S)) t_(S) being the time of occurrence of a firstcharacteristic event of said at least one S-shaped curve (62) of thefocal error signal (FES); T being the time of occurrence of a secondcharacteristic event of the same S-shaped curve (62) of the focal errorsignal (FES); t_(C)=t_(CR)−t_(CS) being the time interval between saidcharacteristic events of said data signal curves (72; 73); herein themeasured parameter value (α) is compared with a predetermined referencevalue (α_(REF)), and wherein it is decided that the optical disc is of afirst type (CD) if the measured parameter value is larger than saidreference value (α_(REF)), and that the optical disc is of a second type(DVD) if the measured parameter value is smaller than said referencevalue (α_(REF)).
 24. Method according to claim 23, wherein: said firstcharacteristic event is a maximum value of the S-shaped curve (62) ofthe focal error signal (FES); said second characteristic event is aminimum value of the same S-shaped curve (62) of the focal error signal(FES); and each of said characteristic events of said data signal curves(72; 73) being the peak of the corresponding curve (72; 73) of the lowfrequency part of the data signal.
 25. Method for recognizing type (CD;DVD) of an optical disc, wherein the distance (D) between two reflectivelayers of the optical disc (2) is measured by a method in accordancewith claim 7; wherein the measured distance is compared with apredetermined reference value; and wherein it is decided that theoptical disc is of a first type (CD) if the measured distance (D) islarger than said reference value, and that the optical disc is of asecond type (DVD) if the measured distance (D) is smaller than saidreference value.
 26. Method according to claim 25, wherein said distance(D) corresponds to the thickness of the disc, and wherein saidpredetermined reference value is preferably in the order of about 0.9mm.
 27. Method according to claim 7, wherein said distance (D)corresponds to the thickness of the disc.
 28. Disc drive apparatus (1),designed to perform a method according to claim
 1. 29. Disc driveapparatus (1), designed to perform a disc type recognition methodaccording to claim 14; wherein the disc drive apparatus (1) is adaptedto handle one disc type only, and wherein the disc drive apparatus (1)rejects an inserted disc if the disc type recognition procedure revealsthat the inserted disc is not a correct disc.
 30. Disc drive apparatus(1), designed to perform a disc type recognition method according toclaim 14; wherein the disc drive apparatus (1) is adapted to handle atleast two different disc types, and wherein the disc drive apparatus (1)proceeds with handling an inserted disc in accordance with the disc typeas revealed by the disc type recognition procedure.